1. Field of the Invention
The present invention relates to reel lawn mowers which mow a lawn on a golf course, garden or square and more particularly to a reel lawn mower which has a reel cutting unit connected to a main body with a prime mover such as an engine or a motor.
2. Description of Related Art
Some reel lawn mowers have a main body with an engine and a reel cutting unit separate from the main body. See JP-A-2005-328838 and U.S. Pat. No. 7,191,584. The reel cutting unit includes a pair of front and rear rollers in its running direction with a spiral cutting reel between the rollers. The cutting reel is rotated by the engine of the main body as a power source to mow a lawn by contact with a bedknife provided in the reel cutting unit. In mowing, the pair of rollers determine the height of the cutting reel from the ground surface and fix the height of grass cutting by the cutting reel and bedknife. The height of the cutting reel from the ground surface can be changed to set a desired grass cutting height, by adjusting the height of at least one of the pair of rollers, for example, the front roller located forward in the running direction. See Paragraph 0010 of JP-A-2005-328838 and the 1st to 7th lines of the 9th section of U.S. Pat. No. 7,191,584.
A lawn mower in which a reel cutting unit is connected to a main body enables pitching and rolling of the reel cutting unit with respect to the main body.
For example, in the lawn mower described in JP-A-2005-328838, pitching and rolling motions of the reel cutting unit (designated by reference sign 14 in JP-A-2005-328838) can be made by a rotation mechanism 16 shown in FIG. 1 (see Paragraph 0009 of JP-A-2005-328838). However, JP-A-2005-328838 only describes that the reel cutting unit 14 is indirectly connected to the main body 12 through a rotation mechanism 16 and does not describe further details.
On the other hand, U.S. Pat. No. 7,191,584 describes a structure which enables pitching and rolling of a reel cutting unit (reel cutting unit 18) with respect to a main body (frame 4) in more detailed form (see the 13th line of the 9th section to the 4th line of the 11th section and FIG. 3 and FIGS. 7 to 10 of U.S. Pat. No. 7,191,584). Specifically, in the lawn mower described in U.S. Pat. No. 7,191,584, a pair of support arms 38 are located on both sides of a reel cutting unit and a pipe crossbar joint 36 is rotatably located between the support arms 38. In this lawn mower, the main body is connected to the crossbar joint 36 through a pair of links 42 to connect the main body and reel cutting unit. The pair of links 42 are connected to the main body and crossbar joint 36 rotatably around an axis parallel to the running direction of the lawn mower, thereby enabling rolling of the reel cutting unit. Pitching of the reel cutting unit is achieved by rotation of the crossbar joint 36 with respect to the pair of links 42.
The pitching and rolling motions of the reel cutting unit with respect to the main body are very important in order to enable the reel cutting unit to follow the ground surface undulation during its running and keep the height of the cutting reel above the ground surface constant to stabilize the grass cutting height. However, the lawn mower described in JP-A-2005-328838 has a serious problem about the rolling motion of the reel cutting unit. The problem is explained below referring to FIGS. 21A to 21C and 22A to 220. In FIGS. 21A to 21C and 22A to 22C, GR represents a turf ground surface and RC represents a cutting reel.
As shown in FIGS. 21A to 21C, in the lawn mower described in JP-A-2005-328838, the reel cutting unit rotates around the rotation mechanism (designated by reference sign 16 in the document), so rotation center RA of the reel cutting unit during rolling is above the cutting reel RC. With the rotation center RA in such a position, when the ground surface GR is inclined right upward (FIG. 21B) from the horizontal level (FIG. 21A) or left upward (FIG. 21C), rolling of the reel cutting unit may cause the rising portion of the cutting reel RC to break into the sod and cause bite S. The direction of inclination of the ground surface GR (right upward and left upward) is the direction as seen from the front of the lawn mower.
Theoretically, however, “bite” seems unlikely to occur since the pair of rollers of the reel cutting unit determine the height of the cutting reel from the ground surface.
During rolling, since the reel cutting unit rotates around the rotation center RA above the cutting reel RC, it moves toward the direction of upward inclination of the ground surface. This causes the portions of the rollers of the reel cutting unit in contact with the upwardly inclined ground surface to break into the ground GR, which reduces the clearance there between the ground surface GR and cutting reel RC and finally causes bite S into the sod. Particularly in the case of a golf course green, since the grass cutting height must be in the range of 2.0 to 5.0 mm and the clearance between the ground surface GR and cutting reel RC should be very small, bite S is likely to occur when the ground surface is inclined considerably.
The lawn mower described in U.S. Pat. No. 7,191,584 claims to suppress the cutting reel RC's bite S into the sod as mentioned above (see the 31st to 39th lines of the 10th section of U.S. Pat. No. 7,191,584). For that purpose, as shown in FIGS. 22A to 22C, rotation center RA (“a focal point F” in U.S. Pat. No. 7,191,584) of the reel cutting unit in rolling is sot at the same level as the height of the bedknife (“a bedknife 24” in U.S. Pat. No. 7,191,584) in contact with the cutting reel RC (see the 17th to 19th lines and 42nd to 65th lines of the 9th section of U.S. Pat. No. 7,191,584). In this respect, U.S. Pat. No. 7,191,584 also described that the rotation center RA (“focal point F”) of the reel cutting unit may be on the ground surface GR or slightly below the ground surface GR (see the 40th to 52nd lines of the 10th section of U.S. Pat. No. 7,191,584).
When the rotation center RA of the reel cutting unit in rolling is at the same level as the height of the bedknife in contact with the cutting reel RC, it is true that bite S into the sod as illustrated in FIGS. 21B and 21C can be prevented.
However, bite S into the sod is prevented in this way only in the initial state in which the rotation center RA of the reel cutting unit is at the same level as the height of the bedknife or slightly below it. The present inventor has found that as a lawn mower like the one described in U.S. Pat. No. 7,191,584 deteriorates over time in which the rotation center RA of the reel cutting unit is at the same level as the height of the bedknife, bite S into the sod may occur at the lowered portion of the cutting reel opposite to its portion lifted by rolling of the reel cutting unit RC (see FIGS. 22B and 22C). An investigation of the reason has revealed that due to deterioration over time the rotation center RA of the reel cutting unit gradually goes down from the height of the bedknife and finally goes down further from the ground surface GR (see the arrows in FIGS. 22A, 22B, and 22C). If the rotation center RA of the reel cutting unit is below the ground surface GR, the reel cutting unit in rolling will move toward the direction of downward inclination of the ground surface. This causes the portions of the pair of rollers of the reel cutting unit in contact with the downwardly inclined ground surface to break into the ground surface GR, which reduces the clearance there between the ground surface GR and cutting reel RC and causes bite S into the sod.
What is the reason that due to deterioration over time the rotation center RA of the reel cutting unit gradually goes down from the height of the bedknife and finally becomes below the ground surface GR? It is lapping of the cutting reel RC. As the cutting reel RC is used overt time, its cutting edge becomes dull, so its original sharpness must be restored by frequent lapping. As a result, the diameter of the cutting reel RC gradually decreases. When the diameter of the cutting reel RC gradually becomes smaller, in order to keep the initial clearance between the ground surface CR and cutting reel RC, the height of at least one of the rollers of the reel cutting unit, for example the front roller in the running direction, must be decreased by about one half of the amount equivalent to the decrease in the diameter of the cutting reel RC. In the process of decreasing the height, there occurs a phenomenon that the rotation center RA of the reel cutting unit becomes below the bedknife. If this process is repeatedly carried out, the rotation center RA of the reel cutting unit will further go down from the ground surface GR.
The present invention has been made in view of this problem and has an object to prevent the cutting reel's bite into the sod which may occur depending on the ground undulation while the reel cutting unit is rolling.